Preparation of C5-/C6-olefins

Abstract

A process for preparing C 5 /C 6 -olefins from an olefinic starting stream comprising C 4 -hydrocarbons comprises a) carrying out a metathesis reaction in the presence of a metathesis catalyst so as to convert the 1-butene, 2-butene and isobutene present in the starting stream into a mixture of C 2 -C 6 -olefins and butanes, b) firstly fractioning the resulting product stream by distillation to give a low boiler fraction A comprising C 2 -C 4 -olefins and butanes or C 2 -C 3 -olefins, which is discharged, and a high boiler fraction comprising C 4 -C 8 -olefins and butanes, c) fractioning the high boiler fraction from b) by distillation to give a low boiler fraction B comprising butenes and butanes, an intermediate boiler fraction C comprising pentene and methylbutene and a high boiler fraction D comprising hexane and methylpentene, d) where all or part of the fraction B and/or C are recirculated to the process step a) and the fraction D is discharged as product.

We claim:  
     
       1. A process for preparing C 5 -/C 6 -olefins and propene from steam cracker or refinery C 4  streams, comprising the substeps 
       (1) removal of butadiene and acetylenic compounds by, optionally, extracting butadiene with a butadiene-selective solvent and subsequently or alternatively selectively hydrogenating butadienes and acetylenic impurities present in the residual steam cracker or refinery C4 streams to give a reaction product comprising n-butenes and isobutene and essentially no butadienes and acetylenic compounds,  
       (2) removing isobutene by either oligomerization or polymerization, or by reacting the reaction product from the preceding stage with an alcohol in the presence of an acid catalyst to give an ether, and separating off the ether and the alcohol either simultaneously with or after the etherification to give a reaction product comprising n-butenes and optionally oxygen-containing impurities, where the ether formed is discharged or redissociated to obtain pure isobutene and the etherification step can be followed by a distillation step for separating off isobutene, where, optionally, C 3 -, i-C 4 - and C 5 -hydrocarbons, introduced in a FCC fraction, are removed by distillation of the ether,  
       (3) optionally removing the oxygen-containing impurities from the raffinate II stream from step (2) by treating the raffinate II stream with adsorber materials,  
       (4) carrying out a metathesis reaction of the raffinate II stream from step (3) in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements so as to convert the 1-butane, 2-butene and isobutene present in the raffinate II stream into a mixture of C 2 -C 6 -olefins and butanes,  
       (5) firstly fractionating the product stream of step (4) by distillation to give a low boiler fraction A comprising C 2 -C 4 -olefins and butanes or C 2 -C 3 -olefins, which is discharged, and a high boiler fraction comprising C 4 -C 6 -olefins and butanes,  
       (6) subsequently fractionating the high boiler fraction from step (5) by distillation to give a low boiler fraction B comprising butenes and butanes, an intermediate boiler fraction C comprising pentene and methylbutene and a high boiler fraction D comprising hexene and methylpentene,  
       (7) where all or part of the fractions B and/or C are recirculated to the process step 4) and the fraction D is discharged as product.  
     
     
       2. A process as claimed in  claim 1 , wherein the C 5 -C 5 -olefins and propene are obtained from a FCC plant or in the dehydrogenation of butane. 
     
     
       3. A process as claimed in  claim 1 , wherein the metathesis reaction is carried out in the presence of heterogeneous metathesis catalysts selected from the group consisting of transition metal compounds of metals of groups VIb, VIIb and VIII of the Periodic Table of the Elements applied to inorganic supports. 
     
     
       4. A process as claimed in  claim 3 , wherein the metathesis catalyst used is rhenium oxide on γ-aluminum oxide or on Al 2 O 3 /B 2 O 3/ SiO 2  mixed supports. 
     
     
       5. A process as claimed in  claim 1 , wherein the substep of selective hydrogenation of butadienes and acetylenic impurities present in the steam cracker or refinery C4 streams is carried out in two stages by bringing the steam cracker or refinery C4 streams into contact with a catalyst comprising at least one metal selected from the group consisting of nickel, palladium and platinum on a support, at from 20 to 200° C., a pressure of from 1 to 50 bar, a volume flow of from 0.5 to 30 m 3  of fresh feed per m 3  of catalyst per hour and a ratio of recycle to feed stream of from 0 to 30 at a molar ratio of hydrogen to diolefins of from 0.5 to 50 to give a reaction product in which, apart from isobutene, the n-butenes 1-butene and 2-butene are present in a molar ratio of from 2:1 to 1:10, and in which essentially no diolefins and acetylenic compounds are present. 
     
     
       6. A process as claimed in  claim 1 , wherein the substep of butadiene extraction from crude C 4  fraction is carried out using a butadiene-selective solvent selected from the group consisting of acetone, furfural, acetonitrile, dimethylacetamide, dimethylformamide and N-methylpyrrolidone to give a reaction product in which the n-butenes 1-butene and 2-butene are present in a molar ratio of from 2:1 to 1:10. 
     
     
       7. A process as claimed in  claim 1 , wherein the substep of isobutene etherification is carried out in a three-stage reactor cascade using methanol or isobutanol in the presence of an acid ion exchanger, in which the extraction mixture flows from the top downward through a flooded fixed-bed catalyst, where the reactor inlet temperature is from 0 to 600° C., the outlet temperature is from 25 to 85° C., the pressure is from 2 to 50 bar and the ratio of isobutanol to isobutene is from 0.8 to 2.0, and the total conversion corresponds to the equilibrium conversion. 
     
     
       8. A process as claimed in  claim 1 , wherein the substep of isobutene removal proceeds by oligomerization or polymerization of isobutene starting from the reaction product obtained from the above-described step of butadiene extraction and/or selective hydrogenation and in the presence of a catalyst selected from the group consisting of heterogeneous catalysts comprising an oxide of a metal of transition group VIb of the Periodic Table of the Elements on an acidic inorganic support to produce a stream having a residual isobutene content of less than 15%. 
     
     
       9. A process as claimed in  claim 1 , wherein the steam cracker or refinery C4 streams are subjected to a first substep of feed purification which is carried out over at least one guard bed comprising a member selected from the group consisting of high surface area aluminum oxides, silica gels, aluminosilicates and molecular sieves.